1. Field of the Invention
The present invention relates to novel polysaccharide biopolymers and to the preparation of polysaccharides, notably Xanthan gum. The subject modified polysaccharides are especially useful in the enhanced oil recovery from partially depleted deposits.
2. Description of the Prior Art
The homo- and heteropolysaccharides, or biopolymers obtained from the fermentation of a carbohydrate by the action of bacteria of the genera Xanthomonas or Arthrobacter, or fungi belonging to the genus Sclerotium, are widely used on an industrial level due to their thickening and viscosity-enhanced properties.
One of the known applications of these polysaccharides of Xanthan gum type is in the secondary and tertiary recovery of oil. In this application, aqueous solutions diluted to a concentration of approximately 300 to 3,000 ppm are used to drive the oil from partially depleted reservoirs. In fact, Xanthan gum is characterized by high viscosity at low concentration, great insensitivity to salinity and to the nature of the salts, and great stability against mechanical stresses. However, solutions prepared from industrial grades, whether directly from the fermentation broth, or prepared by diluting the powder which is precipitated and separated from the broth, have the major disadvantage of rapidly clogging the pores of the rock formations into which they are injected, thus giving rise to undesirable increases in pressure and preventing any further rapid oil recovery. It is known to this art that such clogging originates (i) from the presence of insoluble particles such as cell debris and dead bacteria emanating from the fermentation on the one hand, and (ii) from a number of translucent molecular aggregate deemed microgels, especially if the solution is prepared utilizing a biopolymer which has been precipitated from the fermentation broth, on the other.
Several methods have been proposed to this art for improving the viscosity and/or the filtrability and injectability of such polysaccharide solutions, including heat treatments, addition of flocculants, enzymatic treatments, whether combined or otherwise with a filtration step, for example, filtration through diatomaceous earth or by means of ultrafiltration membranes.
The heat treatment may be carried out either in a neutral or alkaline medium as described in U.S. Pat. Nos. 3,555,447, 3,591,578, 3,729,460 and 4,182,860, or in an acid medium as described in published French Patent Application No. 2,551,070. The treatment at an acid pH under the conditions set forth in said French Patent Application No. 2,551,070 makes it possible to increase the viscosity of the solutions and to significantly improve their filtrability and their injectability. However, it has been determined that this improvement remains inadequate to permit their use as a fluid for injection into porous media having low permeability, e.g., a permeability of less than approximately 1 darcy.
Xanthan and similar heteropolysaccharides contain D-glucose, D-mannose units and glucuronic, pyruvic and acetyl radicals in proportions which vary depending upon the specific strain used and the conditions of fermentation.
It also is known to this art that alkaline treatment removes the acetylated groups. Such process of deacetylation has been described in U.S. Pat. No. 3,000,790 which indicates that deacetylated Xanthan enables the provision of viscous solutions which are less sensitive to mineral salts than the native Xanthan. U.S. Pat. No. 3,964,972 describes the preparation of modified heteropolysaccharides by treatment in an alkaline medium at a high temperature. According to this letter patent, the deacetylation takes place almost immediately and it is probable that hydrolysis and depolymerization are the predominant reaction mechanism.
The pyruvate groups of Xanthan have been removed by heat treatment in the presence of oxalic acid at pH=3 and the acetyl content has remained unchanged. [Carbohydr. Res., 76, 277-80 (1979)]. By the process described in U.S. Pat. No. 4,182,860, in a saline medium, physically and chemically modified polysaccharides are prepared which contain approximately 20% less pyruvate groups and approximately 10% less acetyl groups than the unmodified native polysaccharide, resulting in improved filtrability. However, the loss of the pyruvate groups may cause a modification of the rheological behavior of the final solutions.